CO₂ Sorption on PEI‐Impregnated Mesoporous Silica for Direct Air Capture and Subsequent Conversion to Methanol

Amine-impregnated mesoporous materials are studied for CO₂ sorption at conditions of direct air capture (DAC). Mesoporous silica supports with identical specific pore volume of 0.8 cm³ g⁻¹ but varying pore width (7 – 50 nm) were impregnated with polyethyleneimine (PEI) of different molecular mass (600 – 2000 g mol⁻¹) and loading (20 – 43 wt.-%). CO₂ adsorption was evaluated under dry conditions (450 ppm CO₂) across temperatures from 30 to 70°C, distinguishing the contributions of physisorption and chemisorption. The highest uptake of 1.19 mmol g⁻¹ was observed for sorbents with 30 nm pores and 33.3 wt.-% PEI loading at 50°C, balancing accessible amine density and free pore volume. A temperature-dependent shift from chemisorption to physisorption was identified, particularly in larger pores, where increased polymer mobility promotes enhanced physisorption of CO₂ via the “molecular basket” effect. Long-term cycling showed that narrow pores enhance stability by anchoring PEI through hydrogen bonding, especially for low-molecular-mass PEI. A kinetic model incorporating diffusion and surface reactions reproduces experimental trends across multiple temperatures and confirms that morphology, not intrinsic kinetics, governs CO₂ uptake. This provides mechanistic insight for designing improved sorbents for low-temperature CO₂ sorption under DAC conditions as well as catalysts for subsequent conversion to methanol.